Showdown as Mousavi breaks cover

Iranians are bracing for a tense political encounter today as the opposition leader Mirhossein Mousavi makes his first official public appearance in weeks at Friday prayers at Tehran University.

The reformist's overwhelming defeat in last month's election to the hardliner incumbent President Mahmoud Ahmadinejad provoked the biggest upheaval in Iran since the 1979 revolution, leading to a vicious crackdown by the security forces, the arrest and detention of hundreds of leading activists, journalists and academics. The official death toll from the upheaval stands at 20 but human rights groups believe hundreds may have died.

Mr Mousavi has barely been seen in public since the protests petered out, but today he is expected to attend a sermon delivered by Akbar Hashemi Rafsanjani, his chief clerical supporter and the financial backer of his campaign.

"I will join the lines on Friday as I feel obliged to respond to the call of companions on the path to protecting rights to a noble and free life," Mr Mousavi said in a statement on his website.

After weeks of political stalemate, the event could provide the backdrop for a dangerous showdown as the combined reformist forces within the theocracy square up to the conservative hardliner Mr Ahmadinejad and his religious backers, chief among them the Supreme spiritual leader Ayatollah Ali Khamenei.

The Supreme Leader, who has traditionally been a neutral figure, alienated himself from many Iranians by validating the election results. While Mr Mousavi is a figurehead for reform, several leading religious figures who despise Mr Ahmadinejad and strongly disapprove of his handling of the vote and its aftermath have also called the poll illegitimate. Mr Rafsanjani is in this faction as is the reformist ex-president Mohamed Khatami who is also expected to attend today's prayer service.

Alternatively, today could – some analysts believe – signal the start of a phase of political reconciliation aimed at healing divisions within the clerical hierarchy which has cast the regime into its biggest crisis in 30 years. Mr Rafsanjani's sermon, his first in two months, will be closely watched for any sign that he wants to show emollience towards Ayatollah Khamenei.

An olive branch would be interpreted as an attempt to end the rift with the Supreme Leader, perhaps by depicting the Rafsanjani camp as Islamically purer, more faithful to the tradition of Ayatollah Khomeini, founder of the Islamic Republic, than Mr Ahmadinejad.

Wary of a show of strength by Mr Ahmadinejad's opponents, his supporters warned the opposition yesterday not to create a "provocation" with the Friday sermon. "The vigilant Iranian nation must be aware that [the] sermon should not turn to an arena for undesirable scenes," the Intelligence Minister, Gholamhossein Mohseni-Ejei, said.

Mr Ahmadinejad himself, possibly seeking to distract attention from the internal crisis by provoking further tensions internationally, launched a fierce attack on the West yesterday, warning he would not back down on Iran's nuclear ambitions and blaming Iran's "enemies" for stoking up a rebellion.

"I have told the enemies ," he told supporters in Mashdad, "that this nation... will strike you in the face so hard you will lose your way home". But he also sacked Gholamreza Aghazadeh, the head of Iran's nuclear programme. Mr Aghazadeh is an ally of Mr Rafsanjani.